Mixer-modulator



Oct. 14, 1958 wso ET AL 2,856,586

MIXER-MODULATOR Filed Aug. 13, 1953 2 Sheets-Sheet 1 .36 #I g Y 1766. [IV/9V6 GEN.

INVENTORS 54M 6. NEH/$0M Jay/v m 6241 2 Sheets-Sheet 2 :1 EB. 15 T H Tic]. ED

E. G- NEWSOM ET AL MIXER-MODULATOR V E a.

I EA

1 h TiEI.EE.

Oct. 14, 1958 Filed Aug. 15, 1953 N W W U w MIXER-MODULATOR Earl G. Newsom, Thornwoud, and John W. Gray, Pleasantville, N. Y., assignors to General Precision Laboratory Incorporated, a corporation of New York Application August 13, 1953, Serial No. 373,962

9 @laims. (Cl. 332-43) This. invention pertains to mixer-modulator for com bining two electrical quantities to produce an output quantity. More specifically, this invention pertains to apparatus for amplitude modulation to produce an output quantity having frequencies which are the sum and the difference of the input frequencies, the input frequencies themselves being suppressed.

The process of modulation is necessarily preceded or accompanied by the introduction or mixing of two or more electrical quantities into a single electrical circuit, followed by a synthesis therefrom of a single electrical quantity by an operation which mathematically is multiplication and which produces output energy having frequencies equal to the sum and difference of the input frequencies, as well as higher frequencies. Usually the output energy also contains the input frequencies.

In one common method of modulation employing a pentode or a pentagrid converter tube one alternating or varying input voltage is applied to the first grid and the other input voltage is applied to the third grid. The output contains sum and difference and higher frequencies, and also contains both of the two input frequencies, which may be removed by filtering.

Several other methods of modulation exist which to some degree eliminate the input voltage frequencies from the output voltage, but in each case they either eliminate the input voltages imperfectly, require very exact balancing, or employ complicated circuits.

The instant invention provides a simple mixer-modulator which inherently eliminates both input frequencies from the output without the use of filters of any kind. The elimination of input frequencies is so superior, in fact, to most other methods which as a practical matter merely reduce them in intensity, as to constitute a qualitative improvement.

The invention provides, for both mixing and modulation, two pentode tubes in a bisymmetrical circuit, with inputs to the control and suppressor grids and output from the two plates connected together. Balance is inherently good and for all except themost exacting requirements no balance adjustments are necessary.

The principal purpose of this invention is, then, to provide a simple mixing-modulating circuit which produces an output from which both input frequencies are excluded.

A more specific purpose is to provide a two-tube mixermodulator in which the tubes are alternately made nonconductive, resulting in the elimination of the input modulation frequency from the output energy.

Another purpose is to provide a circuit for the modulation of a square-wave carrier voltage by a modulating signal voltage to produce an output voltage containing modulation product frequencies but not containing either input frequency.

A further understanding of this invention may be secured from the detailed description together with the accompanying drawings, in which:

Figure 1 is a schematic drawing of the circuit of this invention.

Figures'ZA, 2B, 2C, 2D, 2E, 2F, and 26 are graphs illustrating the operation of the invention.

Figure 3 is a schematic drawing illustrating circuit refinements for the highest accuracy.

Referring now to Fig. 1, two similar pentode tubes 11 and 12 have their anodes, screen grids and. cathodes connected in parallel. The anodes 13 and 14 are connected through a common anode resistor 16 to a source of positive potential which may be, for example, 250 volts. The screen grids l7 and 18 are connected through a relatively low resistance common resistor 19 to the same positive voltage source, and are grounded for alternating current voltages by being connected to ground through a relatively large condenser 21. The two cathodes 22 and 23 are grounded through two resistors 24 and 26 connected in series. The twocontrol grids 28 and 29 are connected through equal resistors 31 and 32 to a terminal 27 at the juncture of resistors 24 and 26, the proportions of the resistors 24 and 26 being such as to bias the control grids 28 and 29 to operate on the straight part of the tube characteristic curve. The suppressor grids 33 and 34 are likewise connected to terminal 27 through equal resistors 36 and 37.

One of the two input voltages is required to have an accurate rectangular wave shape. This alternating current input is constant and may be termed the carrier voltage input. It is applied to the two suppressor grids 33 and 34 in push-pull, and must have a voltage magnitude sufiicient to cut off the pentode anode current completely during each negative half cycle. For example, using type 6AS6 tubes, the peak-to-peak carrier value should be at least 10 volts. The carrier frequency may be of any desired magnitude suitable for use with the circuit components, that is, from near zero frequency to several megacycles per second. This carrier voltage may be secured from any conventional source, such as a multivibrator, represented by the rectangle 38 in the figure.

The other input voltage may be termed the modulating voltage. This second input voltage may have any frequency. either higher or lower than that of the carrier voltage input, it need not be harmonically periodic but can vary in any manner, that is it may have any waveform whatever, and may have a bandwidth of any reasonable width, such as 10% or 15% of the central frequency. This modulating input voltage is applied between the control grid 28 and ground, the other control grid 29 being grounded through a condenser 39, this being the conventional manner of single-end feed of a differential amplifier. Alternatively, however, if desired the modu' lating input voltage may be applied to the two control grids 23 and 29 in push-pull, the capacitive ground 39 then being omitted. The magnitude of this modulating voltage should be limited to the linear part of the tube characteristic to secure linearity in the output.

If desired the modulating voltage may be derived from two separate alternating voltages. Their frequencies can be different and there is no limitation on either the frequency or phase relation between them. Likewise there is no limitation on their amplitudes or amplitude relation so long as operation on the linear portion of the tube characteristic is preserved. These two alternating voltages are applied to the two control grids 28 and 29 respectively, and the effective modulating voltage is then the difference between them.

In place of an alternating modulating voltage, a directcurrent voltage may be applied between the two control grids 28 and 29. The output of the circuit is then an alternating voltage representative of the magnitude of the direct voltage.

When modulating voltage is applied to the input terminals 41, and through the coupling condenser 42 to the control grid 28 so as to vary the voltage relation to its cathode 22, the voltage relation of the other control grid 29 to its cathode 23 is thereby made to vary reciprocally. Let it be supposed that the voltage of the control grid 28 is made to rise above its direct current bias value. This increases cathode current, increasing the voltage of both cathodes 22 and 23, but as the voltage of the grid 29 is fixed, the voltage difference between grid 29 and cathode 23 is reduced so that the cathode current in tube 12 falls by about the same amount that the cathode current in the tube 11 has risen. That is to say, the differential amplifier action is such that the signal potential existing between the grid 29 and its associated cathode always varies equally and oppositely to that existing between the grid 28 and its cathode and the respective anode-cathode currents vary accordingly.

Since the screen grids 17 and 18 are connected to the positive-source through a relatively low resistance, the screen grid voltages remain substantially fixed and equal at all times, and the foregoing action is not affected by any changes in screen grid potentials.

The square-wave voltage from the generator 38 is applied in push-pull manner to the suppressor grids 33 and 34, as stated, so that when the grid 33 is negative, the grid 34 is positive, and vice versa. These voltages are illustrated in Figs. 2A and 2B. The voltage during each negative half cycle impressed on each suppressor grid is of such magnitude as to cut off all of the anode current of that tube, and the voltage of each positive half cycle is sufficiently positive to produce maximum anode conductance of a respective tube providing a sufficient positive potential is also applied to the associated control grid. That is, the rectangular voltage shape on the suppressor grids alternately permits the anode current of each tube to be turned on.

Thus if a suppressor grid is made negative it controls conduction and prevents anode current passage without regard to what potentials may exist on the other electrodes, but the reverse is not true. That is, when a suppressor is positive, anode current does not necessarily flow, but flows only under control of the control grid.

Let it be assumed that the modulation input voltage at terminals 41 is sinusoidal. Let it be further assumed that the carrier frequency is exactly five times that of the modulation frequency. Such modulation voltage applied to the control grid 28 is represented in Fig. 2C and similar voltage of opposite polarity, representing that which would be applied to the control grid 29 in the pushpull input case, is depicted in Fig. 2D. When the carrier voltage impressed on the suppressor grid 33 is negative, as at a in Fig. 2A, no current flows in the anode circuit of tube 11 Fig. 1. At the same instant the suppressor grid 34 is positive as represented at b in Fig. 2B, so that current may flow in the anode circuit of tube 12. This current, however, is under control of the grid 29 which, as indicated in Fig. 2D, is effectively at a negative point c, permitting less than average anode current to flow, as is depicted at d in Fig. 2F.

So long as the carrier voltage remains in this phase the anode current in the circuit of tube 12 changes in accordance with the sinusoidal change of the effective voltage of its control grid 29, varying in Fig. 2F from d to 2. At this time the carrier wave changes phase and the suppressor grid 33 becomes positive, as at f, Fig. 2A, and the suppressor 34 becomes negative as at g, Fig. 2B. The current in the anode circuit of tube 12, therefore falls tozero as at h, Fig. 2F, while current starts to flow in the anode circuit of tube 11 under control of its control 'grid 28. As the control grid 28 is at the positive point i, Fig. 2C, the anode current is greater than average, and is illustrated in Fig. 2E at 7. During this carrier half cycle the current in the anode circuit of tube 11 follows the curve of Fig.2E under control of the grid 28 to the point k, when the anode current of tube 11 is again cut;

4 off. It is obvious that the current in the anode resistor 16 is the sum of the anode currents of the two tubes, and thatthe voltage at the junction 43 varies inversely there with in accordance with Ohms law. The voltage at the output conductor 44 is therefore similar except that the direct-current component is eliminated by the series condenser 46 and this voltage is illustrated in Fig. 2G.

The output voltage, represented by the curve of Fig. 26, contains frequencies which are the sum and difference of the input frequencies and higher frequency modulation products. This output voltage, however, does not contain any of the input frequencies if the circuit be ideal, that is, if the bisymmetry be perfect and the tube characteristic curve actually linear. It is apparent that as illustrated in Fig. 26 the carrier frequency phase reverses every half modulation frequency period, so that on the average the carrier frequency cancels out. Also, assuming the suppressor grid voltages equal and constant at any value, the modulation input produces no output because the circuit acts as if it were a differential amplifier, every increase of current at one anode being accompanied by a decrease of the same amount at the other anode so that the sum is constant.

The elimination of the two input frequencies from the output is perhaps more clearly demonstrated by mathematical analysis. The Fourier expansion of a rectangular carrier wave form such as that in Fig. 2A is The process of modulation being a multiplication, as may easily be shown by an analysis of the action within the pentodes of Fig. 1, the process is represented mathematically by multiplication of Equations 1 and 2 resulting in E Sin w t This expression containing only sum and difference terms of w and m establishes that neither the carrier frequency nor the modulation frequency appear as a frequency com ponent of the mixer-modulator output. In the case of a modulation input having any other wave form, such wave form can be broken down to its harmonic content and the above reasoning can be applied to each frequency component separately.

Obviously if the circuit of Fig. 1 be not perfectly bisymmetrical, that is, is not perfectly balanced, some of the input frequencies will be found in the output voltage. Fig. 3 illustrates an arrangement wherein absolute balance may be obtained if desired. In this modified circuit two voltage dividers 47 and 48 and one center-tapped resistor 49 are added to permit perfect manual balancing of the tubes, so that, if the several resistors are exactly matched, no input frequencies exist in the output voltage. Adjustment of the voltage divider 47 balances the tubes 11 and 12 with respect to the modulating voltage input at terminals 41, and adjustment of voltage divider 48 equalizes the effect on the output of the carrier voltage input.

It is obvious that if a push-pull modulating signal input voltage is available, such input can be applied to the suppressor grids of Fig. 1 or 3, with the rectangular carrier voltage applied to the control grid or grids. With proper regard to the difference in tube characteristics of the control grid and the suppressor grid, the circuit can then be made to operate to produce an output voltage from the anodes containing the modulation product frequencies,

and, as heretofore described, suppressing either input voltage frequency.

What is claimed is:

1. A mixer-modulator for modulating a rectangular wave input voltage by a second input voltage of any wave form to obtain an output signal in which the input signal frequencies are suppressed comprising, a pair of tubes each including at least one anode, cathode, control grid electrode and suppressor grid electrode, said cathodes being connected to a terminal of reference potential through a common cathode resistor, said suppressor grids being equally biased for symmetrical operation relative to said terminal of reference potential, said control grids being equally biased for symmetrical operation relative to said terminal of reference potential, circuit means for impressing said rectangular wave input voltage on said suppressor grid electrodes equally but in opposed phase relation to render said tubes alternately non-conductive, circuit means for impressing said second input voltage on a control grid electrode of one of said tubes, the control grid electrode of the other of said tubes being connected through a bias resistance to said terminal of reference potential for equal reciprocal excursions and also being connected through a condenser to a fixed alternating voltage reference terminal, an impedance common to the anode circuits of said tubes, and means for deriving the modulated output signal from said impedance.

2. A mixer-modulator as defined in claim 1 including means for adjusting the relative potentials of said cathodes.

3. A mixer-modulator as defined in claim 1 including means for adjusting the potential of at least one of said suppressor grid electrodes.

4. A mixer-modulator switch for modulating a rectangular wave input voltage by a second input voltage of any wave form to obtain an output signal in which the input signal frequencies are suppressed comprising, a pair of tubes each including at least anode, cathode, control grid electrode and suppressor electrodes, said cathodes being connected to a terminal of reference potential through a common cathode resistor, said suppressor grids being equally biased for symmetrical-opera: tion relative to said terminal of reference potential, said control grids being equally biased for symmetrical operation relative to said terminal of reference potential, circuit means for impressing said rectangular wave input voltage on said suppressor grid electrodes equally but in opposed phase relation to alternately condition said tubes in two states in alternating succession, one of said states being a state of infinite resistance and the other of said states being a state in which anode current depends upon control grid potential, circuit means for impressing said second input voltage on a control grid electrode of one of said tubes, the control grid electrode of the other of said tubes being connected through a bias resistance to said terminal of reference potential and through a condenser to a fixed alternating voltage reference terminal for equal reciprocal excursions, an impedance common to the anode circuits of said tubes, and means for deriving the modulated output signal from said impedance.

5. A mixer-modulator switch as defined in claim 4 including means for adjusting the relative potentials of said cathodes.

6. A mixer-modulator switch as defined in claim 4 including means for adjusting the potential of at least one of said suppressor grid electrodes.

7. A mixer-modulator switch for modulating a rectangular wave input voltage by a second input voltage of any wave form to obtain an output signal in which all input frequencies are suppressed comprising, a pair of tubes each including at least an anode, cathode, and first and second control grid, electrodes, said cathodes being connected to a terminal of reference potential through a common cathode resistor, a pair of equal resistors connecting said second control grid electrodes to said reference potential terminal biasing said grids equally to secure symmetrical operatioirna pair of equal resistors connecting said first control grid electrodes to said reference potential terminal, means for impressing said rectangular wave input voltage on said second control grid electrodes equally but in opposed phase relation to condition each of said tubes in two states in alternating succession, one of said states being a state of infinite resistance conditioned by negative grid voltage beyond cutoff, and the other of said states being a state in which anode current depends only upon first control grid potential conditioned by highly positive second control 1 grid voltage bias, circuit means for impressing said second input voltage on the first control grid electrode of one of said tubes, the first control grid electrode of the other of said tubes being connected through a bias resistance to said terminal of reference potential for equal reciprocal excursions and additionally connected through a condenser to a fixed alternating voltage reference terminal, an impedance common to the anode circuits of said tubes, and means for deriving a modulated output signal from said impedance.

8. A mixer-modulator switch as defined in claim 7 including means for adjusting the relative potentials of said cathodes.

9. A mixer-modulator switch as defined in claim 7 including means for adjusting the potential of at least one of said second control grid electrodes.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES QST for May 1951, pages 11-15 (figure on page 13). 

